Because mammalian placentas are plentiful and are normally discarded as medical waste, they represent a unique source of medically-useful stem cells. There is a need in the medical field for improved compositions and methods of treating amyotrophic lateral sclerosis. The disease is not curable, and the only Food and Drug Administration (FDA)-approved drug treatment is riluzole (RILUTEK®). Other than supportive care, no other therapies exist. As such, provided herein are placental stem cells, and compositions comprising placental stem cells, useful in the treatment of ALS, and methods of using the same to treat ALS.